The present invention relates to enclosures which shield their exterior environments from electro-magnetic radiation that is generated within the enclosures, and more specifically to shielded enclosures for use in X-ray imaging apparatus which house an X-ray tube.
The X-ray imaging apparatus includes a vacuum tube, having a cathode and an anode, which emits X-rays when properly biased. The cathode comprises a tungsten thermionic emitting source and focusing surfaces. The cathode assembly of an X-ray tube typically also includes a filament that heats the cathode to an temperature at which it emits electrons. Upon the application of a bias voltage potential across the anode and cathode, the thermionically emitted electrons traverse the vacuum gap between the cathode and the anode, impacting the anode and thereby generating X-radiation. X-ray tubes used for medical diagnostic imaging are operated at very high anode-cathode voltages, typically 40,000 to 150,000 volts.
This range of operating voltages produces intense electric fields in the vacuum between the anode and the cathode. Such fields are intensified by sharp edges and particles on the surface of the electrodes. If the electric field intensity becomes high enough, a high voltage instability, or discharge, occurs which partially vaporizes the irregularity that produced the high field intensity. If the new surface following the vaporization is not smooth enough to sufficiently lower the electric field intensity, the process randomly repeats until the surface will support the high voltage for an extended period of time. A newly manufactured tube must be "seasoned" by intentionally producing the discharges under controlled conditions. These discharges, also called "tube spits", occur occasionally throughout the life of an X-ray tube providing a means by which the tube cleans itself.
Unfortunately, the high voltage discharges excite the natural resonances of the electrical circuits inside the tube casing. The resulting high frequency oscillations, typically in the range of 100 megahertz, are conducted by cables into the high voltage power supply for the tube an by other electrical connections. From these connections the high frequency signals radiate into electronic equipment in the vicinity of the X-ray apparatus. These oscillations often have very high power which commonly cause the electronic equipment to malfunction and may permanently damage sensitive electronic components.